1. Field of the Invention
This invention relates to microlensed FPAs and more specifically to a method of configuring the detector array and ROIC for sub-pixel de-selection to improve operability.
1. Description of the Related Art
A photodetector registers photon flux striking it as a function of time. An array of a large number of photodetectors can simultaneously register the photon fluxes from many spatial points (called pixels) to form an electronic version of an optical image. In such arrays, some detectors will often be inoperable due to randomly located material defects such as structural dislocations and processing defects that cause electrical shorts or unacceptably high tunneling currents, resulting in poor image uniformity. A detector is typically considered operable if its sensitivity (signal-to-noise ratio, or some other parameter such as quantum efficiency, noise, or dark current or a combination of these parameters) is within a given multiple, say 2-3×, of the mean sensitivity of the array. A defective or bad detector will typically be much worse than this.
Smaller photodetectors reduce the impact of defects which are typically localized in the material, since the detectors cover a smaller area of the material layer and have a reduced chance of intersecting a local defective region. However, smaller detectors also have reduced optical collection efficiency. Focal Plane Arrays (FPAs) use microlenses which concentrate incident radiation into a detector region, thereby permitting smaller area detectors without significant loss of optical collection efficiency. (see Motamedi, et al, FPA's and Thin Film Binary Optic Microlens Integration, Miniaturized Systems with Micro-Optics and Micromechanics, Volume 2687, January 1996, page 70). Using this technique, operability in the 90-98% range has been achieved for strategic Very Long Wavelength Infrared (VLWIR) FPAs operating at 40K. Still, for many applications including hyperspectral imaging using VLWIR FPAs, even 90-98% operability and also clusters of bad pixels may be deemed unacceptable because either spatial or spectral information in a critical band may be lost. These applications may demand high operability, generally 99% or greater, which is presently not achieved even with the use of microlens arrays.
U.S. Pat. No. 6,660,988 to Lee, et al describes an FPA array, and method for fabricating an FPA array, having a plurality of detectors per pixel. The FPA has a microlens array which is custom designed to focus and direct radiation to the most operative detector in each pixel. Each detector is connected to a separate and selectable input of a multiplexer (MUX) or readout integrated circuit (ROIC). Since FPAs are generally tested before the fabrication of the microlens array, the operability of each detector can be evaluated, and a file generated specifying detector operability for each pixel. This file is used to generate a custom microlens mask for fabricating the array using photolithography, in which the shape of the lens for each pixel is chosen to direct the lens focus spot to the best detector in each pixel. The better detector is similarly selected for readout by the MUX or ROIC while the other detectors are not selected. Lee's approach reduces the impact of both material and fabrication defects, since detectors which are defective for any reason can be screened out. Thus, for an array having two detectors per pixel, if the probability of any one detector being defective is 2-10%, the probability of both detectors per pixel being defective is approximately 0.04-1%.
However, Lee's approach requires a custom microlens mask set that is determined by the most operative detector selection for each FPA, which is expensive. In addition, should any of these most operative detectors fail subsequent to microlens application, the pixel will be defective. Furthermore, once the initial testing is done and the microlens is fabricated, it is not possible to recalibrate the FPA to change the selected detector in each pixel.